Guanosine 3', 5'-cyclic phosphate (cGMP) is known to be the second messenger of natriuretic peptides and nitric oxide (NO). To investigate the involvement of natriuretic' peptides in the regulation of the feto-placental circulation, specific radioimmunoassays were used to measure the concentrations of atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP) and cGMP in the umbilical venous plasma of normal and asphyxiated newborns. The plasma concentrations of ANP, BNP and cGMP in asphyxiated newborns were 48-3±12-9pM, 24-5±9-4pM and 4-4±l-6nM (meanis.e.m., n = 10), respectively. These values were significantly higher than those in the normal newborns (17-4±l-9pM, 4-7±l-0pM, and 0-78±0-14nM, respectively). Moreover, the expression of both ANP-A and ANP-B receptor, biologically active receptors for natriuretic peptides, was detected in term human placenta by Northern blot analysis. The expression of natriuretic peptide receptors was further confirmed by binding assay using [125I]-labelled ANP and solubilized crude membrane preparations of placental tissue. These findings suggest that cGMP is produced in the placenta, at least partly, by the action of ANP and BNP secreted from fetal heart, in pathophysiological conditions such as fetal hypoxia.
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